1. Technical Field
The present invention relates to a method of determining (1) the astigmatic power and (2) the power for an intraocular lens, for a toric intraocular lens to be inserted after extracapsular extraction in cataract surgery.
2. Background Art
In cataract surgery by extracapsular extraction, the nucleus of the crystalline lens is first removed by expression and an intraocular lens, which is an artificial lens, is then inserted into the posterior chamber for correction.
However, in the case of a patient with preoperative corneal astigmatism (which is referred to briefly as preoperative astigmatism) due to a disease, for instance, this preoperative astigmatism remains after operation.
To correct this astigmatism, it is common practice to use spectacles or hard contact lenses (hereinafter referred to briefly as HCL) or perform a refractive surgery such as relaxing incision.
However, spectacles and HCL are generally not effective enough to correct an astigmatism in excess of 3 diopters (diopter: hereinafter referred to as (D)) and, moreover, the very use of them is not comfortable to the patient.
Furthermore, the procedure of relaxing incision has the following disadvantages.
(1) In a patient who has undergone corneal incision and suturing in cataract surgery, a reoperation at the same site involves considerable technical difficulties.
(2) Since this operation is tantamount to the intentional creation of a trauma, the risk of corneal infection is increased.
(3) There is a risk for inducing an irregular astigmatism.
(4) This procedure, following cataract surgery, imposes considerable mental and physical burdens on the patient.
(5) It has been criticized as an ethically questionable conduct to intentionally injure the healthy cornea.
(6) While the astigmatic power varies from one individual to another, any correction made by the surgeon is based on his sensory or subjective judgement and may not be thorough correction.
It is for these reasons that the procedure of correcting an astigmatism by relaxing incision has not been commonly practiced.
Heretofore, several procedures for correcting corneal astigmatism with an intraocular lens have been proposed (e.g. U.S. Pat. No. 4,277,852 and Japanese laid-open Utility Model Publication 61-156,915. While this literature suggest that astigmatism can be corrected with an intraocular lens, it is reticent about an important question, namely how, in applying an intraocular lens to the patient, the spherical power and astigmatic power values are to be determined.
There has accordingly been proposed an intraocular lens for correcting the astigmatism induced during operation (hereinafter referred to briefly as operative astigmatism) which is predicated on a prediction of the degree of the corneal astigmatism which occurs as the geometry of the cornea is altered by incision of the sclerocorneal region and subsequent suturing (U.S. Pat. No. 4,512,039). With this intraocular lens, the operative corneal astigmatism can be corrected. However, since the preoperative astigmatism remains uncorrected, correction with spectacles or HCL is still necessary.
Furthermore, while this operative astigmatism is of the order of 0.5(D) at 3 to 6 postoperative months, there have been advances in surgical procedure to the extent that, in most patients, the operative astigmatism is completely eliminated in about one year after operation. Therefore, the incidence of operative astigmatism is not a serious problem today. The most important task is rather correction of preoperative astigmatism.